How to create semicircle from sharp edge
Assuming you want to cut a hole in the shape of a semicircle, you can do it easily by erasing the two faces and adding a Subsurface modifier.

Then add two control edgeloops on the sides you want to be flat.



Alternatively, if want a drawn shape instead of a hole, just select one of the radial vertices and use the Spin modifier. The current position of the 3D Cursor will be used as the center.
Adjust settings like angle and number of segments as desired.